Apple's iTunes online music service has been referred to the European commission after a complaint that it overcharges British customers.

Competition authorities in Brussels will ask Apple why iTunes tracks cost 15% more to download in Britain than in France or Germany.

If Apple is found guilty of breaching European competition law, it faces a maximum fine equalling 10% of worldwide turnover: around £430m. But legal experts said Apple would probably just be ordered to bring British prices in line with the rest of Europe.

An iTunes track costs 79p in Britain, but €0.99 (68p) in France and Germany, and 99c (51p) in the US. British consumers can only download from iTunes sites in cheaper territories if they have a bank account registered there.

Philip Evans, principal policy adviser at Which?, the consumer body that made the original complaint, said: "The online music market is a huge growth area. The single market should work the same in this market as in others ... We'll take on any company, or group of companies, that seek to carve up the market to their benefit."

The government's consumer watchdog, the Office of Fair Trading, dealt with the original Which? complaint. Yesterday it said it had passed the case on to the commission because it affected more than three member countries.

Apple declined to comment yesterday, but it previously told Which? that downloads in Britain were more expensive because the economic model of the record industry in this country was different.

"Look at the price of CDs in the US versus the UK," it said. "We believe the real comparison to be made is with the price of other track downloads in the UK."

The service has sold 150m tracks around the world, but its dominant position has attracted controversy. Its British launch in June saw a boycott by independent record labels which said big labels were offered better deals. A compromise was eventually reached.

Around 70% of the global online music market is controlled by iTunes, but it faces growing competition in the UK. BT, the easyJet founder, Stelios Haji-Ioannou, and Tesco are all launching rival services. Established competitors to iTunes include MyCoke Music, HMV and Microsoft's MSN.

Napster, the former scourge of record companies and now a legitimate online service, operates the second largest online record store in the UK. It is charging 99p a track and £9.95 for a monthly subscription.